Carbon nanotubes (CNTs) have been proposed for use in a number of applications that can take advantage of their unique combination of chemical, mechanical, electrical, and thermal properties. In many instances, these properties can be tailored to the requirements of a particular application by adjusting any combination of carbon nanotube length, diameter, chirality, functionality, and like structural features. Various difficulties have been widely recognized in many applications when working with individual carbon nanotubes. These difficulties can include, but are not limited to, poor solvent solubility, limited dispersibility in composite matrices, inadequate purity, and the like. Without being bound by any theory or mechanism, it is believed that many of these issues can arise due to the strong van der Waals forces that occur between individual carbon nanotubes, thereby causing them to group into bundles or ropes, as known in the art. The extreme aspect ratio of carbon nanotubes can also lead to physical entanglement that can further contribute to these difficulties. The foregoing issues and others can often result in lower than anticipated property enhancements and/or inconsistent performance when individual carbon nanotubes are employed in a chosen application. Although there are various techniques available for de-bundling carbon nanotube ropes, bundles or agglomerates into individual, well-separated members, many of these techniques can detrimentally impact the desirable property enhancements that pristine carbon nanotubes are able to provide. In addition, widespread concerns have been raised regarding the environmental health and safety profile of individual carbon nanotubes due to their small size. Finally, the cost of producing individual carbon nanotubes may be prohibitive for the commercial viability of these entities in many instances.
One carbon nanotube form that has often been proposed for use in many applications is a freestanding, thin layer of carbon nanotubes, commonly referred to in the art as a carbon nanotube mat or a “buckypaper.” Such carbon nanotube mats are often prepared by filtering a fluid dispersion of individualized or dis-aggregated carbon nanotubes onto a suitable collection medium. After filtration is complete, the mat can be peeled away from the collection medium as a freestanding carbon nanotube layer. However, carbon nanotube mats formed in this manner and others often have a low bulk density of less than about 0.4 g/cm3, and often less than 0.1 g/cm3. The low bulk density can pose issues for many downstream applications, particularly those taking advantage of the electrical conductivity of the carbon nanotubes, since the most desirable properties of the carbon nanotubes are often not fully expressed in such structures. Due to their method of production, the carbon nanotubes in carbon nanotube mats are usually randomly aligned with respect to one another, thereby further negating the beneficial property enhancements that can sometimes be afforded by aligned carbon nanotubes. A further issue with the randomly aligned carbon nanotubes in carbon nanotube mats results in these entities being fairly rigid, due to their carbon nanotubes being interpenetrating with one another in all three dimensions in a monolithic construct. Although carbon nanotube mats can be prepared by other techniques with some control over their carbon nanotube alignment, these techniques are not believed to provide the needed throughput to achieve commercial viability. As a further difficulty, surfactants used in producing individualized carbon nanotubes can often be difficult to completely eliminate from the carbon nanotube mat, thereby further eroding the beneficial properties of the carbon nanotubes.
In view of the foregoing, production of carbon nanotube layers that address at least some of the difficulties noted above would be highly desirable. The present disclosure satisfies the foregoing needs and provides related advantages as well.